This application is related to co-pending U.S. patent application Ser. No. 08/612,689 filed on Mar. 8, 1996, entitled xe2x80x9cMethod and Apparatus for Vector Modulation in a Communication Systemxe2x80x9d, by inventor John C. Thomas, assigned to the assignee of the present application, and which is incorporated herein by reference.
1. Technical Field
This invention relates generally to communications systems, and more particularly to voltage controlled oscillators used in communications systems.
2. Background Art
Communication systems, including land-line and wireless communication systems, are increasingly based on digital transmission technologies. This is because digital transmission technologies are capable of providing higher bandwidth, lower distortion, and lower error rates than corresponding analog technologies. A number of digital standards have been adopted, including the Cellular Digital Packet Data (CDPD) standard for cellular telephones, the Frequency Division Multiple Access (FDMA) standard for land-line telephones, etc.
In a digital communication system, such as in a digital modem, it is necessary to modulate a baseband signal to carry the desired digital information on a high-frequency radio wave carrier. This is accomplished by, for example, modulating a baseband signal which serves as an input to CDPD radio circuitry. The base band channel frequency for CDPD transmission is approximately xc2x115kHz, i.e. CDPD transmission has a channel bandwidth of about 30 kHz.
Several prior art modulating methods utilize an algorithm known as Gaussian Minimum Shift Keying (GMSK) waveform synthesis. The use of GMSK waveform synthesis for vector modulation using a finite state machine and a waveform stored in read-only memory (ROM) is known. With such a system, a ROM is used to store sine and cosine values for the phase angles of a rotating phasor. The closest phase angle is determined, and the sine and cosine is read from the ROM to create a pair of control signals for a vector modulated radio transmitter.
A problem with the prior art GMSK waveform synthesis methods is that very large ROMs are required, increasing the size and cost of the circuitry. This is because many sine and cosine values are often required to obtain the desired resolution. In addition, the determination of the phase angle tends to be computationally intensive, and therefore slow and expensive. More particularly, prior GMSK baseband vector modulation systems have been burdened with relatively complex mathematical operations and large amounts of ROM necessary, for example, to map a digitized rotating phasor, rotating with variable angular velocity, into a Cartesian coordinate system. In addition, an accumulator is required in the prior art to store the current phase angle. The known literature is silent as to the use of GMSK techniques for certain other modulation technologies, such as a voltage controlled oscillator (VCO) modulation.
A problem inherent in prior art vector modulation systems is obtaining a faithful reproduction of the waveforms stored in ROM. This is because only a finite number of increments of sine and cosine values can be stored in the ROMs.
Therefore, there is an inherent trade-off between ROM size and resolution in prior art vector modulation systems.
The present invention utilizes the symmetries present in the baseband control signal to a VCO to reduce the mathematical complexity and the amount of ROM to generate a VCO modulated waveform. The present invention therefore allows for less complex (and therefore less expensive and more reliable) finite state machines and for less ROM to be used than was possible in the prior art. Using less ROM has the desirable effect of reducing part size and increasing memory access speed. Furthermore, a faithful reproduction of the waveforms stored in ROM is accomplished by allowing only those digital values in the ROM which are closest (in a digital sense) to the actual modulated waveforms, effectively providing a finer resolution for the waveform. Also, by utilizing GMSK techniques with VCO modulation, a less expensive and less complex alternative for vector modulation is provided.
A radio modem in accordance with the present invention includes a VCO controller receptive to a data stream, where the VCO controller develops a VCO control signal by GMSK waveform synthesis. The radio modem also includes a transmitter having a VCO portion having an input responsive to said GMSK waveform synthesized VCO control signal, such that the transmitter can develop a modulated RF signal carrying the data stream.
A VCO modulator controller of the present invention includes ROM memory storing a number of waveform maps, and a counter coupled to the ROM memory and capable of developing a sequence of ROM addresses. The controller also includes a temporal bit generator responsive to a data stream to develop a next bit Nb, a current bit Cb, and a past bit Pb. Control circuitry of the controller develops a digital waveform signal from selected waveform maps in the ROM memory using the Nb, Cb, and Pb bits and the sequence of ROM memory outputs. Finally, a DAC converts the digital waveform signal to an analog VCO control signal that encodes said data stream.
A method for providing a modulated control voltage in accordance with the present invention comprises the steps of: (a) storing a number of waveform maps in a memory; (b) generating a number of temporal bits including a next bit Nb, a current bit Cb, and a past bit Pb from a data stream; (c) developing a series of addresses for the memory to create a sequence of digital outputs; (d) developing a digital waveform utilizing said Nb, Cb, and Pb bits and the sequence of digital outputs of said memory; and (e) converting the digital waveform signal to an analog VCO control signal that encodes said data stream.
The present invention provides a modulated baseband signal for a digital communication system in a fashion that is less computationally intensive and one which requires less memory than similar circuits of the prior art. This is because the large ROMs and the accumulators of the prior art are not required. ROM size is reduced by utilizing the inherent symmetry in the stored waveforms. In addition, the present invention obtains a faithful reproduction of the waveforms stored in the ROM by storing data with a finer resolution (made possible by eliminating symmetrically redundant data), resulting in less distortion to the transmitted signals.
The present invention is also advantageous in that GMSK techniques are being used for voltage controlled oscillator (VCO) modulation of a carrier signal. This VCO modulation is less expensive than vector modulation techniques, and therefore is a viable substitute for low-end applications.
These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings.